Safety control system for fuel burners



Feb. 10, 1948. H. s. JONES 2,435,940

SAFETY CONTROL SYSTEM FOR FUEL BURNERS Original Filed July 29, 1941 7 Sheets-Sheet 1 INVENT OR.

HARRY S. JONES I I ag moo: cunnzm runs A ORNF Feb. 10, 1948.

H. S. JONES SAFETY CONTROL SYSTEM FOR FUEL BURNERS Original Filed July 29, 1941 7 Sheets-Sheet 2 INVENTOR.

BY g g HAW ATT NEY Feb. 10, 1948. H. s. JONES SAFETY CONTROL SYSTEM FOR FUEL BURNERS Original Filed Ju1y,29, 1941 7 Sheets-Sheet 3 5 6 m m m m m Mm W m I IO 4O 6O 70 8O LENGTH OF CONDUOTOR 5l-FEET huuu fi 8.53028 m0 1.52m

O w m m m w w a s w m wn m OKOJ OGQI JO v 0 O 0 O O 0 G O O O 8 6 4 2 3 2 l INVENTOR.

HARRY s. JONES AT RNEY Feb. 10, 1948. H. s. JONES 2,435,940

7 SAFETY CONTROL SYSTEM FOR FUEL BURNERS Original Filed July 29, 1941 '7 Sheets-Sheet 4 INVENTOR.

- HARRY s. JONES ATRNEY Feb. 10, 1948. 'H. s. JONES SAFETY CONTROL SYSTEM FOR FUEL BURNERS Original Filed July 29, 1941 7 Sheets-Sheet 5 .zrrzzzm FIG. 9.

1N VENTOR.

HARRY s. JONES BY ATNEY Feb. 10, 1948. H. s. JONES SAFETY CONTROL SYSTEM FOR FUEL BURNERS Original Filed July 29, 1941 '7 Sheets-Sheet 6 m n F M 8 0 WI. m q

FIG.| l.

INVENTQR.

. HARRY s. JONES TORNEY Feb. 10, 1948. H. s. JONES 2,435,940

SAFETY CONTROL SYSTEM FOR FUEL BURNERS Original Filed July 29, 1941 '7 Sheets-Sheet FIG. l3.

FIG. l4.

INVENTOR.

HARRY S. JONES lay-{J AT RNEY Patented Feb. 1 0, 1948 SAFETY CONTROL SYSTEM FOR FUEL BURNERS Harry S. Jones, East Orange, N. 3., assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Continuation of application Serial No. 404,523,

July 29, 1941.

'lhis an 1946, Serial No. 670,353

42 Claims. (Cl. 158-28) The present invention relates to safety control systems for fuel burners and more particularly to safety control systems includin means for distinguishing between normal and abnormal conditions of combustion.

A general object of the invention is to provide an improved safety control system for fuel burne'rs that shall operate in accordance with the conductivity of a flame of the burner fuel.

A more specific object of the invention is to provide av safety control system of the flame responsive type for fuel burners. and which utilizes n electric discharge device the conductivity of which is changed substantially immediately upon the presence of a flame by utilizing the electrical conductive property of the flame.

which may be set up due to accidental engagement of the electrodes, or established by virtue of carbonization thereof.

Another. object of the invention is to provide an electronic detector circuit responsive to the conditions existing between a pair of electrodes which shall respond in one manner to the presence of a flame and which shall respond in an opposite manner to the absence of a flame or to the presence of a path between the electrodes whose conductance is greater than that of a flame.

Still another object of the invention is to provide such a detector circuit in which the grounded metallic burner tip may be utilized as one electrode and in which the position of the other electrode with respect to the flame is not critical.

Another object of the invention is to provide in an improved flame detector circuit having an electrode adapted to be engaged by the flame means to ensure response of the circuit in a safe sense if the electrode becomes grounded including means to overcome adverse effects due to leakage capacitance in the circuit.

A further object of the invention is to provide an improved safety control system of the flame responsive type which operates in response to a characteristic of a fluctuating current passing through the flame and in which the need for shielding the conductor leading from the system to the flame for appreciable and normal working distances has been eliminated.

plication filed May 17,

Another object of the invention is to provide an improved flame detector circuit which may be supplied with energy from a commercial alternating current supply source and in which the p tential for measuring the flame conductivity shall be of a pulsating unidirectional nature.

A further object of the invention is to provide an improved combustion control system for a plurality of fuel fired furnaces wherein means are provided for simultaneously distinguishing between actual conditions of combustion and simulated conditions of combustion in all of the furnaces.

In combustion control systems which have been proposed heretofore, various means have been employed for determining if combustion conditions are proper, and whether combustion actually takes place, one such means comprising an electrode which extends into the flame of the burner and which is so connected in the system as to provide a conductive path of relatively low resistance to ground through the flame. The variation in the electrical conductivity of this path to ground when a flame is present and when a flame is not present is commonly employed to change the bias on the control grid of an electronic valve for controlling a thermal safety switch. Since there is a possibility that a low resistance path may be set up from the electrode to ground through other agencies than by means of the flame, for example, a low resistance path which may be established between the flame electrode anclground by reason of carbonization, by accidental touching of the electrode to ground, or by other abnormal conditions simulating combustion, provisions have been made in devices of the prior art for preventing the fuel supply and ignition from being turned on when such abnormal conditions exist.

Since an abnormal condition of this character simulating combustion may arise after the system is already in operation, means have been provided for distinguishing between such abnormal conditions and normal combustion while the system is in operation. For example, in a thermostatically controlled house heating system, if the control system is insensitive to the presence of the flame after initial ignition of the flame, the fuel feeding means will be operated continuously as long as the room thermostat is closed. If the flame should then be extinguished, the furnace will be flooded with fuel and a highly explosive mixture of the latter will be permitted to accumulate.

As is well known in the prior art all flames are the result of a chemical reaction. In the case of a gas flame, for example,,the oxy en of the "air combines with the carbon and hydrogen of the gas to form carbon dioxide and water vapor.

, The flame is always accompanied by ionized particles which'are the result of the reaction behydrogen of the gas.

capable of conducting a current between the electrode is held in the outer parts of the flame and that when the movable electrode is moved toward the burner a region is reached where the flame conductivity becomes substantially equal in both directions of current flow between the burner and the movable electrode. By the term "direction of flame propagation" is meant the direction away from the burner. If the movable electrode is moved through this region toward the burner another region is encountered where the conductivity is greater in the direction of flame propagation than in the other direction. This phenomena is believed to be due to the presence of a concentration of positive ions in the base of the flame. The negative ions or electrons or the ionized particles of the flame are much more mobile than the posititve ions and, there fore, tend to accumulate in the outer region of the flame while the positive ions tend to accumulate in the region closely adjacent the burner.

, It has been determined further that by suitably biasing the movable electrode positively with respect to the potential of the burner the tendency of the flame to conduct better in the direcion of flame propagation in the region closely adjacent the burner may be overcome and as a result the position of the'movable electrode in the frame is then not critical. In commercial fuel burner installations equipped with a safety con.- trol system of this type the position of the movable electrode with respect to the flame may change due to accidental displacement of the electrode or to change in the flame size because of changes in fuel supply, drafts and other unpredictable factors, and therefore, an arrangement in which the position of the movable electrode is not critical is particularly advantageous. Therefore, means have been provided in accordance with the present invention to maintain the movable electrode positive with respect to the burner.

In safety control sysems of his type which have been proposed inthe prior art and which operate in response to a characteristic of a fluctuating electric current passed through the flame, it has been necessary to employ an electrostatically shielded conductor to connect the flame electrode to the detector circuit in order to maintain the distributed capacitance to ground of the conductor and the movable or flame electrode smaller than a predetermined value. If the distributed capacitance to ground of that conductor and the flame electrode in the prior art arrangements is greater than this predetermined value,

, the system is rendered unstable or erratic or otherwise unsatisfactory and unsafe in operation.

In accordance with the present invention the need for electrostatically shielding the conductor connecting the flame electrode to the detector circuit has been eliminated by properly propor-- tloning a certain condenser in the detector cirportance.

Another feature of the present invention is the elimination of undesirable and unsafe effects which may be introduced into the system opera-.

tion as a result of stray capacitance between certain of the circuit components and ground. For example, when the flame electrode is connected to the control electrode in an electronic valve in a circuit so arranged that the valve is nonconductive under normal flame conditions and should be conductive under abnormal flame conditions, stray capacitance between various of the circuit elements and ground tends to apply a potential on the control electrode of such polarity as to simulate normal flame conditions even when the control electrode is grounded. It may be desirable in some cases that the circuit be so connected that this stray capacitance effect is eliminated or at least minimized to the end that the electronic valve will be operated in a safe sense upon the occurrence of an abnormal con dition. Provisions for attaining this end have ticularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects attained with its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is a wiring diagram illustrating one embodiment of the present invention;

Fig. 2 illustrates the voltage relations in the input and output circuits of the detector circuit of Fig. 1;

Fig. 3 is a three-dimensional curve illustrating in more detail the operation of the detector circuit of Fig. 1 for all values of flame resistance from zero to infinity;

Fig. 4 is a wiring diagram of a modification of the detector circuit of the arrangement of Fig. 1;

Fig. 5 is a curve showing the maximum distributed capacitance of the conductor connecting the flame electrode and the detector circuit of Fig. 1 and thereby the maximum length of that conductor for a. given value of capacitance in the detector input circuit;

Fig. 6 is a curve showing the relation between the range of flame resistances to which the detector circuit of Fig. 1 is responsive for a given length of conductor connecting the detector circuit with the flame electrode;

Fig. 7 is a wiring diagram illustrating a modification of the arrangement of Fig. 1 in which the combustion conditions in a plurality of burners may be controlled by means of a single control system;

Fig. 8 illustrates a modification of the arrangement of Fig. 7;

' Fig. 9 is a wiring diagram of another modification of the detector circuit of the arrangement of F18. 1;

Fig. 10 is a wiring diagram of still another modification o! the detector circuit of the arrangement of Fig. 1;

Fig. 11 is a curve illustrating the variation of cathode-control electrode potential in the input ircuit of the detector oi Fig. 10 during certain conditions of operation;

Figs. 12 and 13 are wiring diagrams of other modifications of the detector and amplifier circuits oi the arrangement of Fig. 1; and

Fig. 14 is a wiring diagram of another modification oi the detector amplifier circuit of the arrangement of Fig. 1 which may be used when the present invention is applied under severe conditions described hereinafter.

Fig. 1 shows a gas burner l supplied with gas from a conduit 2 and the flow of gas to the burner is controlled by an electrically operated or other suitable Valve 3. A pilot burner d is provided which is controlled by an electrically operated or other suitable valve 5; and means are provided for igniting the pilot flame including a pair of electrodes 6 which are connected to the terminals of the secondary winding 1 of an ignition transformer 8 having a primary winding 9 which is adapted to he energized from the alternating current supply lines L and L The fuel valve operating circuit of my control system is controlled by means of a thermostat to which may be located in a room or space to be heated. The thermostat it? may he of any suitable construction and includes a bimetallic elernent it connected by means of a conductor 92 to alternating current supply line L and a contact arm is adapted to engage a stationary contact it which is connected to alternating current supply line L through a thermal safety switch i5 and the primary winding 86 of a transformer H. I

The thermal safety switch it is Preferably of the form disclosed in the F. S. Denison Patent 1,953,081 which issued on May 8, 1 .234. As illustrated more or less diagrammatically in the drawing, this switch comprises a stationary arm it and a movable arm iii which is biased for movement away from the arm 5 8 but is normally held in engagement with the latter by means of a bimetallic element 2@ Birnetallic element 26 is rigidly secured at one end to a stationary block 28 and is arranged to he heated by a coil 22 when the latter is energized through a circuit which will be described hereinafter. Upon energization of coil 22 for a predetermined period of time the bi= metallic element 2@ will he warped sufiiciently in the counter clockwise direction to permit the arm i9 acting under a spring or other bias to separate from the arm it to thereby interrupt the circuit including the thermostat l and the transformer primary Winding iii. The thermal safety switch l will remain locked in this position until manually adjusted to its normal closed position.

The transformer ll which supplies power for the control system is a combination step-up and step-down transformer and comprises the line voltage primary winding l6, low voltage secondary windings 23 and 24, and high voltage secondary windings 25, 2e and 21. The low voltage secondary winding 23 is connected by conductors 28 to the heater filaments 29 and 39 of an eiec.. tronic valve 3i, and supplies energizing current thereto. The electronic valve 3! is a twin type amplifier valve, for example, a type 6N7, and includes two triodes, designated A and B. in one envelope. For convenience, the triode designated A in the drawing will be referred to hereinafter as the rectifier, and the triode B will be referred .to as the detector.

The rectifier A includes a cathode 32, a control electrode 33, and an anode 34, and the detector B includes a cathode 35, a control electrode 36 and an anode 31. As shown, the cathodes 32 and 35 are directly connected to each other within the valve 3 l The rectifier A is supplied with electrical energy from the transformer secondary winding 25 through a circuit which may be traced from the upper terminal of winding 35 through a conductor 38, anode 34, cathode 32, and a conductor 38 in which a resistor 40 is inserted to the lower ter minal of winding 25. This circuit is conductive only during the half cycles of the alternating voltage supply source When the upper terminal of winding 25 is positive with respect to the lower terminal thereof due to the action of rectifier A. Thus, a pulsating potential drop is produced across resistor 48 in the direction to render terminal 42 of resistor til positive with respect to terminal 45 thereof. The control electrode 33 of rectifier A is directly connected to the cathode 32.

The detector B is supplied with electrical energy from the transformer secondary winding to through an output circuit which may he traced from the lower terminal of winding 26 through a conductor in which a resistor is inserted, anode tl, cathode and a conductor 45 to the upper terminal of winding 26.

The conductivity of the output circuit of de tector B is controlled by an input circuit one branch of which connects the control electrode 36 to the grounded and negative terminal Ci oi resistor Q through a connection including a pair of resistors 66 and ll which are connected in parallel, a conductor ill, a contact 39 and a switch arm at, the latter of which is connected to ground. During the normal operation of the sys tem the switch arm 59 is held away from the contact 49 but is in engagement with the latter at certain times for a purpose-later described.

A second branch of the input circuit of the detector 13 connects the control electrode 38 through conductor hi to an electrode 52 which projects into the flame of the pilot burner t. The pilot burner t is connected to ground,

A third branch of the input circuit of detector B connects the control electrode 355 to the cathode 35 through a parallel connectionincluding condensers 53 and lid in two branches and resistors :26 and ll, in parallel, connected in series with a resistor 55 in another branch. The resistor is normally shunted by a connection which may be traced from the upper end of resistor 55 through the conductor 48, a switch arm 55, a contact ill .and a conductor 58 to the lower end of resistor 55, The reason for providing two parallel connected condensers E53 and 5d and two parallel connteicted resistor 56 and ll is explained hereina er.

The transformer secondary windings 25 and 2'5 are so wound on transformer ll that the detector B is conductive only on the half cycles when the rectifier A is non-conductive and vice versa. Thus, a pulsating direct current potential drop is produced across resistor to during the half cycles when the detector 13 is non-conductive. The phase of this pulsating potential drop is such that it will be at a maximum value when the anode 31 of the detector B is negative, and zero when the anode 31 of detector B is positive, as is illustrated by the dot and dash line 59 in Fig. 2A. In Fig. 2 the anode voltage applied to the detector B is indlcated by the solid line 60. The bias p tential produced across resistor to is indicated by the 7 dot and dash lines 53, and the potential applied between the control electrode 38 and cathode 35 of detector B is indicated by the dotted line 3|.

The pulsating potential drop or biasing potential produced across the resistor 40 is applied to the input circuit of the detector B through the flame, if present. and the polarity thereof is such as to cause a pulsating direct current to flow through the flame in the direction opposite to the direction of flame propagation. This input circuit may be traced from the positive terminal 42 of resistor 40, conductor 39, resistor 55 in series with resistors 48 and 4! in parallel shunted by condensers 53 and 54, conductor 5|, flame electrode 52, the flame resistance, burner 4 to ground, and through ground to the negative terminal 41 of resistor 40.

When a flame is not present at the burner and the contact 49 and switch arm 50 are out of engagement and no other conductive path exists from the control electrode 36 to ground, no bias potential will be applied to the control electrode 36 of the detector B, as is shown in Fig. 2B. The control electrode 36 of detector B will then be at substantially the same potential as the cathode 35 since the control electrode 36 and cathode 35 are connected by resistors 46, 41 and 55. As a result the detector B will then be conductive during the time its anode 31 is positive.

Upon the appearance of a normal flame, the

bias potential produced across resistor 40 is applied to the control electrode 33 through the resistance of the flame. The resulting flow of current through the resistance of the flame produces a potential across resistors 46 and 41 which is stored on the condensers 53 and 54 and is of the proper polarity to apply a negative potential to the control electrode 36 and thereby to reduce the current flow in the output circuit of the detector B. This potential is stored on the condensers 53 and 54 during the half cycle when the rectifier A is conductive and the detector B is non-conductive and is maintained on the condensers 53 and 54 during the succeeding half cycle when the anode 31 is positive, as illustrated by the dotted line 6| in Fig. 2C. This result is accomplished by properly proportioning condensers 53 and 54 and resistors 46 and 41. The result is that little or no current is conducted in the output circuit of detector B during the condition of normal flame.

If a path more conductive than a flame exists between the flame electrode and ground, however, the charge which is stored on the condensers 53 and 54 during the half cycle when the anode 3! is negative will leak ofi through that conductive path during the same half cycle and the initial portion of the succeeding half cycle, as shown in Fig. 2D, to thereby substantially reduce the potential on the condensers 53 and 56 during the succeeding half cycle. Consequently, the potential applied to the control electrode 38 is rendered substantially less negative, and as a result, current will flow in the output circuit of detector B during the intervals indicated in Fig. 2D by the reference character 62.

It will thus be noted that current flows in the output circuit of the detector B when there is no flame and when the flame electrode 52 is grounded, and that little or no current flows in this output circuit when a normal flame is present at the burner.

In Fig. 3 I have illustrated a three-dimensional curve showing the operation of the detector B for all values of flame resistance'irom zero to in- 8 flnity. In Fig. 3 the vertical axis is the negative control electrode to cathode potential of the detector B and has been designated by the reference character eg. The horizontal axis shows the passage of time, and the third axis shows the variation 01' the flame resistance relatively to the effective resistance of the parallel connected resistors 4B and 41. In Fig. 3 the flame resistance has been designated by the reference character RF and the efiective resistance 01' resistors 48 and 41 has been designated by the reference character R. It willbe noted that as the flame resistance RF approaches zero, a condition which exists when the flame electrode 52 is directly connected to ground, the ratio of RF/R approaches the time axis 0X.

When the flame resistance is zero the potential e; which is impressed between the control electrode 38 and cathode 35 of detector B is identical with the potential produced across resistor 40 by therectifier A. This potential is shown by the heavy, full line 63 in Fig. 3. With the condensers 53 and 54 open-circuited, this potential changes as indicated by the curves 64. 55, 3B, and 31, which are shown in the lighter full lines, as the flame resistance increases in magnitude. It will be noted that the amplitudes of curves 53-31 decrease and approach zero as RF increases and approaches infinity. This result occurs because es depends upon the factor When condensers 53 and 54 are connected into the circuit, these condensers hold part 01' the peak potential applied to the control electrode 35 between successive half cycles of the potential applied to the flame, as shown by the dotted curves 68, 69, 13, H, and 12. When the flame resistance RF is very small, the condensers 53 and 54 almost entirely discharge before the next half cycle peak as is illustrated by the curve 63. Above a certain value of the flame resistance RF, however. the condensers 53 and 54 hold substantially all of the peak potential from one half cycle to the next as is shown by the curves ill, H, and I2, Between half wave peaks of potential across the resistor 50, the control electrode potential e;

thus varies from zero to some peak negative value and down again to a low value as the flame resistance RF varies from zero to the normal flame range of resistance values and to values above the normal flame range, respectively. The low value of e when the flame resistance RF is low is caused by the rapid discharge of condensers 53 and 54 through the low value of flame resistance RF. The low value of e when the flame resistance RF is very high results from the fact that the peak negative potential applied to the control electrode 35 is then only a very small fraction of the potential produced across resistor 43 inasmuch as this fraction is determined by the ratio It should be noted that the actual value of resistance between the flame electrode 53 and ground which will operate to render the detector 18 conductive depends upon the value of the capacitance of condensers 53 and 54. As the capacitance of condensers 53 and 5a is increased,

per resistance limit of resistance at which the this type.

detector B is rendered conductive, however. is substantially unaffected by the capacity of condensers 53 and 54 but depends upon the effective value of the parallel resistances l6 and 41. The

higher the effective value of the resistors 08 and 41, the higher the upper resistance limit of the flame at which the detector B is rendered conductive.

It is noted that the distributed capacitance along conductor i and flame electrode 52 tends to decrease the range oi. flame resistance to which the detector B is sensitive, and also tends to render the detector B insensitive to the presence or absence of a flame at the burner t. Most of this effect is produced by the distributed capacitance along the conductor 5| since the flame electrode 52 is ordinarily very short compared to the length of conductor 5|. This capacitance to ground varies in direct proportion to the length of conductor 5|, and therefore, is different for each installation of a safety control system of That it is necessary to provide some means to minimize this adverse effect will be readily apparent. This end is accomplished in the arrangement of Fig. l, and has been accomplished in the prior art devices, by electrostatically shielding the conductor 5| by means of a metallic shield 5 IA. The shield 5|A entirely surrounds the conductor 5| and is connecteddirectly to the cathode 35 of detector B. The efiect oi the metallic shield 51A is to reduce the distributed capacitance of conductor 5| to-ground.

With the conductor 5| electrostatically shielde as shown in the arrangement of Fig. 1, there is practically no limit to the length of conductor 5| that may be used since the shield to conductor capacity merely shunts the condensers 53 and 54 and has no other eifect on the operation of the detector B than to vary the time delay of operation thereof in response to a change in the condition between the flame electrode 52 and ground. For example, when the total capacitance of the condensers 53 and 54 is 0.02 microfarads, 300 feet of '7 millimeter shielded conductor 5|. such as Belden airduct would have no other effect on the operation of the circuit than approximately to double the time delay.

In Fig. 4 I have illustrated more or less diagrammatically a modification of the arrangement shown in Fig. 1 wherein the need for shielding the conductor 5| for appreciable and normal working distances has been eliminated. This advantageous result is attained in Fig. 4 by properly proportioning the total capacitance of condensers 53A and 54A relatively to the distributed capacitance between the conductor 5| and ground.

With the arrangement of Fig. 4 in which the conductor 5i is not electrostatically shielded, there are definite limits to the length of conductor 5| which may be safely used. These limits are such, however, that by proper design the conductor 5| may be of the length required in most, if not all, of the ordinary installations When the capacitance CF is comparable with the capacitance C, or is larger than the latter, a large negative potential exists on the control electrode 38 and is maintained thereon due to the fact that the capacitance CF, the capacitance C and the resistances 48 and 41 are large. Under this condition the detector B is rendered insensitive to the presence or absence of the flame at the burner and also is unable to distinguish between the normal and abnormal conditions between the flame electrode 52 and ground. This is not a safe condition of operation, and therefore, is highly unsatisfactory. Between these two extremes where the ratio of CF/C is zero, or at least is very small, and where the ratio of cF/C is large, there is a zone of instability, that is, a region wherein the operation of detector B is unstable. In this region of instability, the operation of the detector B is unpredictable and the detector B may or may not respond in a safe sense to the presence or absence of a flame at the burner. Safe operation of the arrangement illustrated in Fig. 4 is at tainable only in the range between this region of instability and the extreme where the ratio of CF/C is zero.

In Fig. 5 I have illustrated an experimentally determined curve showing the maximum safe value of CF for a given value of C which may be employed without rendering the operation of the dectector B unstable. Fig. 5 also illustrates the maximum length of unshielded conductor 55 which may be utilized for a given value of C without rendering the detector B unstable in operation.

It is noted that this curve includes a safety factor. That is to say. the maximum value oil CF for any value of C may be slightly greater than that indicated by the curve and still allow the circuit to o erate-safely. For example. for a value of C of 0.01 microfarads (CF may be 0 0013 microfarads instead of approximately 0.0009 microfarads as shown by the curve without rendering the detector B unstable in operation. This operation, however, is somewhat critical, and therefore, for reliable and safe performance the maximum value of CF. and therefore-the maximum length of conductor 5| for a given value of C should not exceed that shown by the curve.

By reference to Fig. 5, it will be seen that the value of C is approximately eleven times as great as the value of CF when C is 0.01 microfarads and that when C is 0.04 microiarads. it is slightly more than five times as great as CF.

It should be noted that the operation of the detector B in the Fig. 4 arrangement is critical and unstable when the value of C is less than approximately 0.0075 microiarads. This is indicated by the peculiar shape of the curve in that region. Therefore, values of C less than 0.0075 microfarads are unsuitable unless the conductor 5| is provided with a shield.

The range of flame resistance to which the detector B is responsive is dependent upon the values of the capacitances CF and C. In Fig. 6 I have illustrated three experimentally determined curves showing the relation between the range of flame resistance to which the detector B is responsive for a given value of the capacitance CF'when the value of the capacitance C is 0.01, 0.02 and 0.04 microfarada' As is shown by these curves the range of flame resistance to which the detector B is responsive decreases as the capacitance CF increases and increases as the capacitance C increases.

The time required for the detector B to operate in response to the presence or absencev of the flame at the burner, or in response to an abnormal condition between the flame electrode 52 and ground, is dependent upon the product of CR, where C is the total capacitance of condensers 53A and 54A and R is the effective resistance of resistors 46 and 41. Accordingly, in choosing particular values of the circuit constants the relative proportions of the capacitance C and the resistance R must be taken into consideration in order to provide proper time delay in the operation of the detector B. Such time delay is desirable in many installations in ,Order to prevent deenergization of the fuel burner control system on extreme and transient fluctuations of the pilot burner flame which may be caused, for example, by opening the furnace door and by fluctuations in the supply of fuel.

It will thus be noted that in the design of a safety control system of the type shown in Fig. 4 in which the conductor-| is not electrostatically shielded from ground, there are at least four factors which must be taken into consideration. These factors are: (1) stability, and therefore reliability of operation of the detector B, (2) the upper limit of resistance between the flame electrode 52 and ground to which the detector B is to respond, (3) the lower limit or "ground out resistance between the flame electrode 52 and ground to which the detector B is to respond, and (4) the time delay in the operation of the detector B to a change in the resistance between the flame electrode and ground.

By way of illustration it has been found that a value of' 0.01 microfarads for capacitance C, and a value of 25 megohms for theeflective resistance of resistors 46 and 41 permits the use of at least 25 feet of unshielded conductor 5| placed within a grounded protective conduit having an inside diameter of one-half inch. With these circuit values, a time delay of approximately one second is obtained. The upper limit of resistance between the flame electrode 52 and ground to which the detector '13 then responds is approximately 150 megohms and the lower limit or ground out" resistance between the flame electrode 52 and ground to which the detector B responds is approximately 0.13 megohms.

The arrangement of Fig. 4 with the values of the circuit components noted operates in a reliable and safe manner and may be utilized in the majority of installations of safety control systems of this type. In most installations the length of conductor 5| required does not exceed 25 feet and the upper limit of flame resistance to which the detector B must respond does not exceed 150 megohms. In fact, the resistance of most flames is of the order of 50 megohms. In addition, a value of 0.13 megohms for the lower limit of ground out resistance between the flame electrode 52 and ground is satisfactory in most installations and a time delay of approximately one second is also desirable. It will be understood that, if desired, longer lengths of unshielded conductor 5| maybe safely used by properly'proportioning the capacitance of condensers 53A and 54A relatively to the distributed capacitance between conductor 5! and ground.

Asillustrated in Figs. 1 and 4, the current conducted by the detector B controls the potential difference" between the terminals of the resistor 44, which potential difference in turn is utilized to control the conductivity of an electronic valve V, The electronic valve V is a power amplifier valve, for example, a type 6V6 and includes a cathode I3, a control electrode 14, a

screen grid 15, a beam forming plate 18 which is connected to the cathode I3, an anode TI and a heater filament 18. For convenience, valve 'V will hereinafter be referred to as the amplifier.

The heater filament I8 is shunted by a fixed I arrangement is such that upon failure of either filament 29 or 30 the current flow through the heater filament 18 will be insuflicient to main-' tain the emission from the cathode I3 and the amplifier V- consequently will become non-conductive. This results, therefore; in operation of the system in a safe sense upon failure of any one of the filaments 29 and 30 or 18 since failure of any one of these filaments causes the emission from the cathode 13 to be insufficient.

The amplifier V is supplied with energy from the transformer secondary winding 21 through an output circuit which may be traced from the lower terminal thereof, as seen in Figs. 1 and 4, through a conductor 80, anode ll, cathode 13, a conductor 8|, conductor 43, transformer secondary winding 26, conductor 85, conductor 58,

. a winding 82 of arelay 83 in parallel with a conaccidental opening of that winding. Opening of winding 26 would otherwise tend to cause the amplifier V to become conductive, but with the connection shown and described opening of the winding 25 causes the amplifier V to become non-conductive.

The conductivity of the amplifier V is adapted to be controlled in accordance with the potential drop across resistor 46, and thereby in accordance with the condition of the path between the flame electrode 52 and ground. To this end, one terminal of the resistor 44 is connected to the amplifier cathode 13 by the conductor 8! and the other terminal thereof is connected to the control elec-= trode it by a conductor 86. These connections including the resistance it are hereinafter termed trol electrode 14 will then become substantially that of cathode l3, and accordingly, the amplifier Vwill become conductive.

For convenience of illustration, amplifier V will be referred to as conductive when its output current is sufiicient to cause the relay 83 to close its normally opened contacts and as non-- conductive when its output current is insuflicient 13 for um pal-dose. Similarly, detector 3 will be referred to as conductive when its output current is sufficient to cause amplifier V to become non-conductive, and will be referred to as nonconductive when its output current is insufficient to cause the amplifier V to become non-conductive. For further convenience, a contact through which a circuit is completed when a relay is energized will be designated a front contact of the relay, and a contact through which a circuit is completed when the relay is deenergized will be designated a back contact.

As illustrated in Fig. l, the relay 88 comprises the winding 82 and switch arms 81 and 88 which are controlled by winding 82 and which cooperate with front contacts 88 and 80, respectively. Switch arms 81 and 88 also cooperate with back contacts 8| and 92, respectively. There are also provided two relays 93 and 80 which are utilized for purposes explained hereinafter. Relay 88 comprises a winding 95 which operates switch arms 86 and 91 in addition to the switch arm 56 previously mentioned. The switch arms 88 and 91 cooperate respectively with front contacts 88 and 89. Relay 90 comprises a winding I which operates a switch arm wt in addition to the switch arm 50 previously mentioned. The switch arm IOI cooperates with a front contact I02.

When the temperature of the space to be heated is higher than the desired value, all parts of the system are in the positions shown in Fig. 1 and the fuel burner is then deenergized. If the tem-' perature of the room or space to be controlled falls below the value it is desired to maintain, the thermostat I0 operates to move the switch blade I3 into engagement with the contact I4. This results in closure of an energizing circuit to the transformer primary winding I6 and thereby eii'ects energization of the transformer secondary windings.

The detector B is not immediately rendered conductive, however, because the switch arm 50 and contact 09 are then in engagement and operate to' apply the full negative potential across the resistor 40- to the control elect"ode 85. Therefore, as soon as the filament I8 of the amplifier V becomes heated, the amplifier V becomes conductive and energizes the winding 82 of relay 83. The relay 82 then operates switch arm 8'! into engagement with the front contact 89 to complete an energizing circuit for the winding I00 of relay 9| in series with heater coil 22 of the thermal safety switch I5. This circuit may be traced from the upper terminal of transformer secondary winding 2 through a conductor I08, winding I00, a conductor I00, heater coil 22, a conductor I05, switch arm 81, contact 89, and a conductor I06 to the lower terminal of winding 2d.

The energization of winding I00 causes the switch arm IOI to close on the front contact I02 to complete a holding circuit for the winding I00. This holding circuit may be traced from the upper terminal of winding 24, through conductor I08, winding I00, switch arm IN, and contact I02 to the lower terminal of winding 20. Winding I00 when energized also actuates switch arm 50 out of engagement with contact 49 thus opening the circuit which applies the negative poten-' tial to the control electrode 35 of detector B, and thereby permitting the detector B to become conductive providing conditions between the flame electrode 52 and ground are proper.

Before the starting cycle is allowed to proceed further. however, a check is made to detect the presence of leakage resistance paths between the electrode 52 and ground, and initiation of the burner flame is prevented in a manner described hereinafter if such leakage paths exist and are of the order of a normal flame resistance or slightly higher, as desired. This desirable feature is obtained by an interlocking of the relays 88 and 84 to the end that the cycle can not proceed further until the relay 83 has been deenergized and then reenergized. If the resistance of the path between the flame electrode 52 and ground is of the order of a normal flame, a charge is stored on the condensers 58 and 54 which maintains the detector B non-conductive and consequently maintains the amplifier V conductive whereby the relay 83 is not deenergized.

The resistor 55 which is connected in series with the resistors 48 and 41 in parallel between the control electrode 38 and cathode 35 of ,detector B isprovided to accomplish this result. At this point of the starting cycle of the system it will be noted that the shunt circuit around the resistor 55 is open at the switch arm 56. The resister 55, therefore. is now eifectively connected in the input circuit of the detector B and operates to place a higher resistance in shunt with the condensers 58 and 54, and therefore, a higher average negative potential is maintained on the control electrode 36 by the condensers 53 and 54 than would be maintained thereon if the resistance 55 were shunted out. The detector B at this point in the starting cycle of the system is therefore sensitive to conductive paths between the electrode 52 and ground of resistance greater than that of a normal flame. If such conductive paths exist, the detector B will remain nonconductive and the system will continue in the condition last described, namely with the relays 83 and 84 energized. The magnitude of the resistance of such leakage conductive paths from electrode 52 to ground to which the detector B is sensitive is determined by the total capacitance of the condensers 53 and 54 and the effective value of the resistances 06, 41 and 55.

If no such leakage conductive paths exist between the flame electrode 52 and ground, however, no potential is stored on condensers '53 and 54, and accordingly, detector B becomes conductive causing the amplifier V to become nonconductive and the relay 83 to be deenergized. This causes switch arm 81 to close back contact 81 to thereby complete an energizing circuit for the winding of relay 93. This energizing circuit may be traced from the upper terminal of transformer secondary winding 24 through conductor I03, winding 95, a conductor I01, contact SI, switch arm ill, conductor I05, heater coil 22, conductor I04, switch arm WI, and contact I02 back to the winding 24.

Energization of the relay 93 causes it to close a holding circuit for itself, causes it to shunt the resistor 55, and causes it to energize the ignition transformer primary winding 9 and the pilot fuel valve 5. The holding circuit for the relay 93 may be traced from the transformer secondary winding 24, through conductor I03, winding 85, switch arm 98, a conductor Hi0, conductor I05, heater coil 22, conductor I04, switch arm IOI and contact I02 back to winding 20. The resistor 55 is shunted by the closure of switch arm 56 on front contact 57 through the conductors 48 and 58. The energizing circuit for the ignition transformer primary winding 9 may be traced from the alternating current supply line L through a conductor 5539. the primary winding 9, contact 92.

switch arm 88, a conductor IIO, contact 99, switch arm 91, and a conductor III to the alter nating current supply line L The energizing circuit for the pilot fuel valve 5 may be traced from the supply line L through conductor I09, valve 5, a conductor II2, contact 99, switch arm 91 and conductor III to the supply line L As a result of the energization of the ignition transformer 8 and the pilot fuel valve 5, a flame should appear at the pilot burner 4. If no flame appears no further action will take place until the system is deenergized by the action of the heater coil 22 on the thermal safety switch I5. If a. flame appears, however, detector B will become non-conductive as previously described, and consequently, amplifier V will become conductive and relay 83 will again be energized.

Reenergization of relay 83 causes the closure of a short circuit around the heater coil 22, opening of the energizing circuit through the ignition transformer primary winding 9, and energization of the main fuel valve 3. The burner I is then in full operation. The shortcircuit around the heater coil 22 may be traced from the left end thereof as seen in Fig. 1 to conductor I05, switch arm 81, contact 89, conductor I06, contact I02, switch arm IN and conductor I04 to the other end of heater coil 22. The energizing circult of transformer primary winding 9 is opened by the movement of switch arm 88 away from the back contact 92. The energizing circuit to the main fuel valve 3 may be traced from the supply line L through conductor I09, fuel valve 3, a conductor H3, contact 90, switch arm 38, conductor I I 0, contact 99, switch arm 9'! and conductor III to the supply line L.

If, after the burner has been placed in full operation, the flame should become extinguished, the detector B will become conductive, amplifier V will become non-conductive and relay 83 will be deenergized, thus causing the main burner valve 3 to close, effecting reenergization of the ignition transformer 8, and causing opening of the shunt circuit around the heater coil 22 of the thermal safety switch I5. If the flame then reappears. the burner will go again into full operation. If the flame does not reappear within a predetermined time, however, the system will be deenergized by the opening of the thermal safety switch I5 due to the action of the heater coil 22.

From the foregoing it will be seen that the system of Fig. 1 is adapted to distinguish between normal flame conditions and abnormal conditions at the burner 4 and operates to deenergize the system if an abnormal condition prevails longer than a predetermined time.

As illustrated in Figs. 1 and 4, two condensers of one-half the nominal value required for the capacitance C are provided to reduce the possibility of unsafe failure of the system due to open circuiting of thecapacitance C. Thus, in the arrangement of Fig. 4, the value of capacitance C may desirably be so chosen that when it is reduced to one-half its nominal value due to open circuiting of one condenser 53A or 54A, the circuit will still fail safe upon flame failure or upon direct connection of the flame electrode -with ground when twenty-five feet of unshielded cable in one-half inch flexible and grounded conduit is used.

The two parallel connected resistors 46 and 41 are each double the-value required for resistance R and are provided to reduce the possibility of unsafe failure of the system in case one resistor is open circuited.

In Fig. 7 I have illustrated more or less diagrammatically a modification of the arrangement illustrated in Fig. 1 which may be employed in checking the combustion conditions of a plurality of fuel burners simultaneously. The circuit arrangement illustrated in Fig.7 will operate upon flame failure, or upon the occurrence of an abnormally low resistance path to ground from any one of the flame electrodes associated with the respective burners due to accidental shorting or carbonization or for any other reason, to close the main fuel supply valve or to maintain said fuel valve closed if any one of the burners fails to ignite.

In Fig. 7, in order to avoid confusion of the drawing, I have shown an arrangement responsive to the combustion conditions of only three fuel burners, but it will be understood that more fuel burners may be so controlled from a single unit if desired.

In the arrangement of Fig. '7 it will be understood that the circuit components for the input circuits of the various detector units B may be so designed as to permit the use of unshielded conductors between the detector units and the flame electrodes associated therewith as described in connection with Fig. 4, or if desired, those conductors may be provided with shields as illussure of a normally open push button H5. The

trated in connection with Fig. 1.

Although it will be understood that the flame responsive units of Fig. 7 may be employed in conjunction with the control relay system of Fig. 1, I have for purposes of simplification shown this form of my invention as adapted to be employed lighted burners. Thus, in Fig. 7 the pilot and main burner flames are manually lighted, and after combustion has taken place, the system operates so as to maintain the pilot and main burner fuel valves in open position so long as normal conditions of combustion exist.

As illustrated the arrangement of Fig. 7 includes a plurality of ignition transformers Ild each of which is associated with a respective pilot burner s and is adapted to be energized together with the pilot valve actuating means upon cloenergizing circuit for the pilot valve may be traced from the alternating current supply line L through a conductor M6 to the push button I15, conductor Ill, the pilot valve 5, and conductor I I8 to the supply line L The energizing circuit for the ignition transformers I It may be traced from the supply line L conductor H6, push button I I5, a conductor I I9, a switch arm I20, contact iZI, a conductor I22, the primary windings of the ignition transformers us which may desirably be connected in parallel, and conductors I 23 and H8 to the supply line L The energizing circuit for the transformer primary winding I6 may be traced from the supply line L through conductor M6 to push button II5, conductors H9 and I20 and the primary winding I6 to the supply line L}.

Thus. when the push button H5 is closed the pilot valve 5 is opened, the ignition transformers H4 are energized, and the transformer primary winding I6 is energized. As soon as a pilot flame appears atany of the burners, the detector B associated therewith isrendered non-conductive. The output circuits of all of the detectors B are connected in parallel and this parallel connection is connected in series with the resistor 44 and the transformer secondary winding 26. In this arrangement as in the Fig. 1

in conjunction with manually Fig. 1 or Fig. 4, depending upon whether a shielded conductor 5| is utilized or not, so that the rendering of the detectors B non-conductive upon the appearance of a flame at each of the pilot burners results in the amplifier V being rendered conductive and thereby in energization of the relay 83. v

In the arrangement of Fig. 7 the relay 83 is adapted to actuate a, switch arm lit": and the switch arm we already mentioned. When the relay 83 is deenergized, the switch arm H0 is in engagement with the contact Hi, and when the relay {is is energized, the switch arms i253 and 525 are moved into engagement with contacts M and ml, respectively.

The switch arm 25 and contact i2"! are included in an energizing circuit to the main valve 3 so that energization of the relay 8% results in the establishment of a circuit for actuating the main fuel valve 3 to its open position. This 3 circuit may be traced from the supply line L-, conductor till, a normally closed push button its, a conductor its, switch armthe, contact Mi, conductor its, main valve 3, and conductor lit to the supply line L2, lhe switch arm no and contact E26 are included in the energizing circuit to the pilot valve 5 independent of the push button energizing circuit so that energization of the relay 83 results in the establishment of a holding circuit for the pilot valve which maybe traced from the supply line L conductor lit, push button iZt, conductor us, a conductor i3i, contact E26; switch arm i2il, conductor M9, conductor ii'l, ,pilot valve 5, and conductor M8 to the supply line L The switch arm tilt and contact I26 are also included in the energizing circuit to the transformer primary winding iii which is independent of the push button energizing circuit so that energization of the relay 83 results in the establishment of a holding circuit for the transformer primary winding 66. This circuit may be traced from the supply line L through conductor H6, push button i28, conductor i29, conductor 53h contact I26, switch arm'iZEl, conductor 12d, and the transformer primary winding iii to the supply line L Furthermore, it will be noted that when the relay 83 is energized the energizing circuit to the ignition transformers lid is inter rupted and thereby the ignition transformers are deenergized as a result of separation of the switch arm [26 and the contact WI. The normal operating condition of the arrangement of Fig. 7 having thus been established the push button H5 may subsequently be released.

The system will then continue to operate since the conditions of combustion are properv so long as it is desired to maintain theburners in operation. For interrupting the energizing circuit to the pilot and main fuel valves when it is desired to shut down the system, means have been provided in the form of the normally closed push button I28. The push button no operates when 18 actuated so as to result in closure or the pilot and main fuel valves thereby shutting oil or the supply of fuel to the various burners and in deenergization of the transformer primary wind inc i8.

As soon as one of the pilot flames is extinguished, the corresponding detector B becomes conductive and the amplifier V is rendered nonconductive whereupon the relay 83 is deenergized. This results in separation of the switch arms 20 and I25 from their respective contacts I25 and I21. If thereafter, the push button $28 is released and later closed, the system will remain deenergized until the push button H5 is closed.

If while the system is in operation one of the electrodes 52 should be accidentally connected to ground through an abnormally low resistance by reason of carbonization or for any other reason, the detector unit individual thereto will immediately detect this condition and operate to cause deenergization of the entire system. The. opera-= tion of each of the detectors B upon the occur rence of such an abnormal condition is identical with that described in connection with either Fig. i or Fig. 4: and hence need not he described in detail.

In Fig. 8 I have illustrated more or less diagrammatically a modification of the arrangement shown in Fig. *2 which has the particular utility in indicating the location of the burner at which an abnormal condition of combustion has arisen. The arrangement of Fig. 8, for purposes of simplification of the drawings, been shown as adapted to be employed in conjunction with mam ually lighted burners, and has not been shown in conjunction with a control system as in the arrangement of Fig. 7, but instead has been shown as being adapted to actuate an audible alarm, such as a bell E32, upon the occurrence of an abnormal condition of combustion at any of the burners.

In order to facilitate the determination of the location of the burner at winch an abnormal condition of combustion exists, means have been provided in this arrangement in the form of a resistor E33 and a neon lamp E33A in shunt there with in the output circuits of each or the detectors B. Thus, when the detector B associated with the burner at which an abnormal condition of combustion exists is rendered conductive as a result of such abnormal condition, the how of output current from that detector 3 through the resistor loll associated therewith produces a potential drop across the resistor B3 of sufficient magnitude to cause the neon lamp 3321 individual thereto to .fiash. It will be apparent that the provision of such signalling means facilitates the determination of which burner is at fault.

In this arrangement the supply of energizing current to the flame detector circuits is controlled by means of a manually operable switch L which is connected between the supply lines L and L and the input circuit of the flame detector circuits. The audible indicating means or hell $32 is adapted to be connected to the supp y lines L and L through the switch L upon the closure of switch arm I25 upon contact (21. When a normal condition of combustion exists at all of the burners, the switch arm I25 is held away from the contact E21 by the relay 83 which is then energized, but upon the occurrence of flame failure or any other abnormal condition of combustion at anyone of the burners, the relay 83 is deene'rgized whereupon the switch arm 25 is actuatedinto engagement with the contact I21 to close the energizing circuit for bell I32. En-

ergization of hell I32 thus audibly indicates to an attendant that an abnormal condition exists at one of the burners and the attendant by noticing which one of the neon lamps I33A is flashing can readily determine which burner is at fault.

In Fig. 9 I have illustrated more or less diagrammatically another modification of the arrangement of Fig. 1 wherein the use of a shield BIA for electrostatically shielding the conductor from ground may be dispensed with and an unshielded conductor 5| utilized instead. More specifically, in Fig. 9 I have illustrated a different form of flame detector circuit which may be utilized and to which the present invention may be adapted to eliminate the need for electrostatically shielding the conductor 5 I In Fig. 9 electrical energizing current forthe detector and amplifier circuit is obtained from a transformer I34 which is a combination stepup and step-down transformer comprising a line voltage primary winding I35, a low voltage secondary winding I36 and a high voltage secondary winding I31. The low voltage secondary winding I36 is connected by conductors I38 and I39 to the heater filaments I 40 and HI of a pair of electronic valves I42 and I43, respectively. The electronic valve I42 is a triode of any suitable type and functions in this circuit as a detector. The electronic valve I43 is a triode of the type commonly known as -a power amplifier valve. The detector I42 includes an anode I44,

.trol electrode I 48 is maintained at a negative potential with respect to the cathode I49 through. out the complete cycle, and accordingly, the amplifier I43 may not conduct suflicient current to energize the winding 82 of the relay 83. Conversely. when the detector I42 is non-conductive, or carrying little if any current, the potential drop across the resistor I5I will be small. any charge on the condenser I52 will leak of! after a few cycles and the control electrode I48 will be at the same potential as the cathode I49. As a result. the amplifier I43 will conduct sufiicient current to energize the winding 32 of relay 83.

It will be apparent that the amplifier I43 responds inversely to the condition of detector I42, that is, when the detector conducts a large current the amplifier conducts a small current and vice versa, as in the arrangements of Figs. 1 and 4.

The control electrode I45 of the detector I42 is connected through a protective resistance I56 and conductor 5i to an electrode 52 which is insulated from the burner 4 and extends into the pilot flame as in the previous arrangements described. The conductor 5| is connected to the conductor I54 through a parallel circuit including a condenser I51 and two resistors I58 and I59 connected in series. A circuit including the switch arm 56 and contact 51 of the Fig. 1 arrangement is provided for shunting the resistance I59. The resistor I59 in this arrangement asin the arrangement of Fig. l, is provided for initially checking the resistance of the circuit a control electrode I45, a cathode I46, and the heater filament I411. The amplifier valve I43 includes an anode I41, a control electrode I48, a cathode I49, and the heater filament I4 I.

The anode circuit of the detector I42 is supplied with alternating voltage from the transformer secondary winding I31 through a circuit,

hereinafter termed the output circuit, which may be traced from the upper terminal of the transformer secondary winding I 31 through a conductor I50, a resistor I5I which is shunted by a condenser I52, a conductor I53, the anode I44, the cathode I46, and a conductor I54 to-the lower terminal of the winding I31. In this arrangement the switch arm 50 and contact 49 are arranged to shunt resistor I5I and condenser I52 for initially permitting the relay 83 to become conductive upon closure of the thermal safety switch blade I3 with contact I4.

The detector I42 is connected to the transformer secondary winding I31 in such a manner that it may conduct current only during those half cycles in which the upper terminal of the transformer winding I 31 is positive with respect to the lower terminal, while the amplifier I43 is connected to the transformer secondary winding I31 so that it may conduct only during alternate half cycles, that is, when the lower terminal of the winding is positive with respect to the upper terminal. The control electrode I48 of the amlifier I 43 is normally connected to the cathode I49 through an input circuit including a protective resistor I55, and resistor I5I and condenser I52 in parallel. When the detector I42 is conductive, a potential drop exists across the resistor I5l, a charge is built up on the condenser I52, and the control electrode I48 is maintained at a negative potential with respect to thecathode I49. The condenser I52 and the resistor I 5I are so proportioned that several cycles must elapse before the charge built up on the condenser can leak oil through the resistor. Therefore, the conpath between the flame electrode and ground for the presence of leakage conductive paths. In the arrangement of Fig. 9 when a flame is established at the pilot burner 4 a circuit is established which may be traced from the upper terminal of the transformer secondary winding I31 through conductor I50, resistor -I5I and condenser I52 in parallel, a protective resistor I66 to ground. the pilot burner 4, the flame resistance, electrode 52, conductor 5| condenser I 51 and resistance I58 in parallel, switch arm 56, contact 51, and conductor I54 to the lower terminal of winding I31. Resistor I59 is shunted out of this circuit through closure of switch arm 56 on contact 51 when a flame is present at the burner 4.

Due to the rectifying property of the flame a charge will be stored on the condenser I51 of the proper polarity to apply a negative potential on the control electrode I45 relatively to the potential of the cathode I46. This reduces the output current of the detector I 42 sufliciently so that the amplifier I43 becomes conductive to thereby efiect energization of the relay 83.

If the flame should become extinguished while the arrangement of Fig. 9 is in operation, the conductive path between electrode 52 and ground will be removed and the control electrode I45 will tend to assume the potential of the cathode I46 making the detector I42 conductive. This will cause the amplifier I43 to become non-conductive, thus deenergizing the relay 83.

If the electrode 52 should accidentally be connected to ground through a low resistance path, the detector I42 will be made conductive since the control electrode I45,will then be at substantially the same potential as the anode I 44. This will cause the amplifier I43 to become non-conductive, thus deenergizing the relay 83.

By properly proportioning the capacitance of an unshielded conductor may be utilized.

when the circuit components I51 and i58 are thus properly proportioned relatively to the length of conductor 5 I, the adverse effects of the distributed capacitance along the length of the conductor 5| are obviated and the circuit arrangement of Fig. 9 is adapted to give reliable and safe operation in response to the conditions of combustion at the pilot burner.

It is noted that with the circuit arrangement dscribed in Figs. 1 and 4 the occurrence of leakage capacity efiects between either or both of the transformer secondary windings 26 and 21 m ground and between windings 26 and 21 and winding 25 tends to produce an unsafe condition when the flame electrode 52 is grounded. The effect of such leakage capacities is to tend to maintain the detector 13 non-conductive when the flame electrode 52 is connected through a low resistance path to ground and thereby to maintain the amplifier V and relay 83 energized thus permitting the supply of fuel to the burner although the flame electrode 52 is grounded. More specifically, leakage capacities between either of the transformer secondary windings and ground operate to filter the pulsating potential produced across resistor 438 by winding 25 and rectifier A. Consequently, when the flame electrode 52 is grounded, such leakage capacities cause a negative potential to be applied to the control electrode 35 of the detector B. This sheet is the same as that produced by a flame, and therefore, is an unsafe condition.

In Figs. 1 and e the tendency of such leakage capacity efiects between the transformer secondary windings 25 and a? and ground and be tween windings 26 and 27, and winding 25 to pro duce an unsafe condition when the flame electrode 52 is grounded has been obviated by properly choosing the resistance of the resistor 5%. I have determined that when the resistance of the re sister it is relatively low compared to the leakage capacity efiects likely to occur between the windings 2e and 2'5 and ground, for example, when the resistance of resistor til is of the order of 500!) ohms, it is impossible for leakage capacity effects of sufficient magnitude to afiect the operation of the system to exist between the transformer secondary windings 25 and 2t and ground. Accordingly, in the arrangements of Figs. 2 and 4 a resistance id of 5000 ohms is utilized.

In Fig. 1G I have illustrated more or less dia= grammatically a modification of the arrangement of Fig. 1 in which the efiect of leakage capacities between the transformer secondary windings 25 and 21 has been eliminated in a different manner and thereby the possibility of unsafe failure of the system avoided. In Fig. 10, elements correspondingto those in the Fig. 1 arrangement have been designated by the same reference numerals.

As illustrated, the Fig. modification differs in structure from the Fig. 1 arrangement mainly in that the rectifier A and resistor 46 have been interchanged with respect to the grounded terminal of transformer winding 25 and the rectifier A has been reversed so as to conduct on the same half cycle that the detector B is conductive. with this circuit arrangement the potential applied to the flame is a distorted alternating potential as illustrated in Fig. 11 whereas the potential applied to the flame in the Fig.

1 arrangement is a pulsating direct current pocathode 35 and control electrode 38 obtained in the Fig. 10 arrangement when the flame electrode 52 is connected to ground. In Fig. 11 the horizontal axis shows the passage of time and the vertical axis shows the potential applied between the cathode 35 and control electrode 38. The positive half waves a-b and c-d are due to the high potential difference between cathode 35 and ground when the rectifier A is non-conductive which is obtained at that time because substantially the whole potential of transformer winding 25 is then applied between cathode 35 and ground. The small half wave b-c is due to the relatively small difference in potential between cathode 35 and ground during the half cycle when rectifier A is conductive since at that time most of the potential of transformer winding 25 is applied across resistor 40. This small potential tends to apply a positive potential on the control electrode 38 with respect to the cathode 35 and is of a magnitude sufficient to overcome a y potential on the condenser in the input circuit of the detector existing because of leakage capacitance efiects in the circuit, and tending to apply a negative potential to the control electrode 36. The wave form between points b and c when such a positive potential exists is shown by the dotted line in Fig. ll.

In Fig. 1d the condensers 53 and 54 of the Fig. i arrangement have been replaced by a. single condenser 5313 which is of relatively smaller capacity so that the time delay in the omration of the relay 8-3 in response to a change in conductivity between the flame electrode 52 and ground efiected by the condenser 53B is negligible. In Fig. 10 time delay in the operation of the relay S3 in response to a change in the conductivity between the electrode 52 and ground is effected by a condenser Edi which is connected in parallel with the resistor id, The effect of condenser.

till is to prevent immediateresponse oi the amplifier V upon change in current flow through the resistor M. The time of delay of that response is determined by the capacity of the con denser Hit and may be adjusted as desired by properly choosing condenser iEi.

The modification of Fig. 10 incorporates a further desirable feature not obtained in the arrangement of Fig. i. In the arrangement of Fig. 1 it has been found that disengagement of the electrode 5'12 from ground following a direct physical engagement of the electrode 52 with ground produces momentary energization of the relay 83. This energization of relay 83 continues for only a few seconds and is not objectionable in most cases, but in those instances where it may be objectionable, it may be avoided in the manner illustrated by Fig. 16 wherein a resistor E62 is provided between the resistance #4 and the anode 31 of the detector B. The efiect of resistor 62 is to temporarily assume the full change in potential upon a; change in the current flow through the resistors E62 and 54 so that substantially no change in potential of the control electrode'l l relatively to the cathode T3 of amplifier V is effected in response to a current surge through the resistances 44 and N52. In order to accomplish this desirable result it is noted that duration of the current surge through the resistors 44 and I62 must be less than the time delay of resistor 44 and condenser ISL In Fig. 12 I have illustrated a modification of the arrangement of Fig. 1 wherein a different arrangement is shown for eliminating the undesirable leakage capacitance 'efiects between the 23 transformer secondary windings 26 and 26 and ground. In Fig. 12, a single transformer winding I63 having a number of taps intermediate its ends is employed in lieu of the separate transformer secondary windings 25, 26 and 21 of the Fig. l arrangement. In addition, in Fi 12 the detector B and the amplifier V have been placed within a single envelope. The rectifier A has been shown separate from the detector B, and in this modification, may be of the copper oxide or any other suitable type. For convenience of illustration the rectifier A has been shown as a diode. Elements in Fig. 12' which correspond to those in Fig. 1 have been designated by the same reference numerals.

In Fig. 12 reference numeral I63 designates a transformer secondary winding having terminals I64 and I65, a fixed tap I66 and an adjustable tap I61. A portion of the winding between taps I65 and I61 supplies energizing voltage to the rectifier A. The portion of the winding between tap I66 and terminal I65 supplies energizing voltage to the amplifier V. The entire winding arm I10 and a contact "I is inserted. In the arrangement of Fig. 12 the switcharm I16 and contact "I are utilized instead of the switch arm 56 and contact 49 of the Fig. 1 arrangement. The switch arm I16 and contact I1I are provided for the same purpose-that the switch arm 56 and contact 49 in Fig. 1 are provided, namely, for the purpose of initially permitting the relay 83 to become conductive upon closure of the thermostat blade I3 with the contact I4.

The adjustable 'tap I61 is connected to re-' sistor 46 through a conductor I.12., If the leakage capacitance effects are negligible it is noted that the tap I61 may be coincident with the terminal I 64. If such effects are appreciable, however, the tap I61 may be moved along the transformer winding I63 until the potential difference between it and terminal I64 is sufficient to overcome any opposing potential obtained as a result of such leakage capacitance efiect. That is to say, the tendency of leakage capacity effects between the transformer secondary winding and ground is to apply a. negative potential to the control electrode 36 with respect to the potential of cathode 35 and can be eliminated by adjusting the tap I 61 along winding I63. The effect of such adjustment is to introduce a potential in thecathode-control electrode circuit of detector B which is in opposition to the potential established on the control electrode 36 as a result of leakage capacitance between the transformer secondary winding I63 and ground and thereby eliminate the undesirable effects of such leakage capacitance.

In Fig. 13 I have shown a modification of the circuit of Fig. 12 wherein a different means is employed for eliminating the undesirable leakage capacitance eifects previously described. The adjustable tap I61 of the Fig. 12 arrangement has been dispensed with, and terminal 42 of resistor 46 is connected permanently to termi- 24 ml I84 of transformer winding I 66. The leakage capacitance efiects are eliminated in the present modification through the use of a resistor I12, connected between terminal 4! of the resistor 46 and terminal I85 of the transformer secondary winding I63. The resistance of resistor I12 is large with respect to that of resistor 46; As a result, the potential across resistor 46 and thereby the potental between control electrode 86 and cathode 36 of detector B when flame electrode 52 is grounded takes on the general wave form illustrated in Fig. 11. During the half cycles izb and cd, rectifier A is conductive, a large current fiows through resistor 46, and the potential drop across its terminals is large. During the half cycle b-c, rectifier A is non-conductive, and current fiows from the transformer through resistor I12 and resistor 46 in series. Since resistor I12 is larger than resistor 46, most of the transformer potential appears across the resistor I12, leaving a relatively small potential as shown between b and'c'in Fig.

11 to appear across resistor 46, By properly proportioning resistors I12 and 46, this potential may be made to assume any desired value. 1

In Fig. 14 I have shown a modification of the circuit of Fig. 1 which may be used where adverse conditions prevail which produce severe carbonization of the electrodes. In such cases, it is desirable to have a system which will distinguish between actual flame conditions and all other conductive paths to ground having comparable resistance. This end is accomplished, in this circuit, by making the detector circuit responsive primarily to the rectifying characteristic of the flame.

The rectifier A, in Fig. 14, is a triode whose control electrode 33 is connected through a conductor I13 to a terminal I14 of relay winding 82. Energizing voltage is supplied to rectifier A from that portion of the transformer winding I63 between terminal I65 and a tap I15. De-

I tector B and amplifier V are both supplied from the transformer winding I63. The separate transformer winding I16 is used to bias amplifier V positively with respect to detector B so that the potential of detector anode 31 may be effective to control the conductivity of amplifier V. Since other means are provided in this circuit for distinguishing between high resistance grounds and actual flame conditions, resistor 55 is omitted. All other circuit elements are the same as in one or more of the circuits previously described, and have been designated by the same reference numerals.

The conductor I13 serves as a feed-back connection from the output circuit of amplifier V to the input circuit of rectifier A. Because of this connection, rectifier A is maintained non-conductive when amplifier V is non-conductive, and vice versa, in a manner to be explained hereinafter. When the burner is being started, rectifier A is, therefore, non-conductive, and detector B is conductive. In this modification alternating current is applied to the circuit path including the flame which may be traced from terminal I64 of winding I63, resistances 46 and 41 in parallel, with condensers 53 and 54, conductor 5|, flame electrode 52, the fiame, and resistance I12 to terminal I65 of winding I63.-

Advantage is taken of the normal rectifying action of the flame to charge condensers 53 and 54 and reduce the output of detector B, thereby causing amplifier V and rectifier A to become slightly conductive. when rectifier A becomes slightly conductive, the charge on condensers It and I4 is increased, and the circuit action becomes cumulative until detector B is substantially cut oil. Since this action cannot be initiated except by the rectifying action of a fiame at electrode 52, the system will not start in operation until a flame appears, and is thus completely insensitive to the presence of a high resistance leakage path to ground at electrode 52.

Specifically, when amplifier V is nonconductive, there is no potential difference across the relay winding 82, and terminal H6 of the relay winding 82 is at substantially the same potential as terminal Hi l of transformer winding I63; Control electrode 33, through its connection with terminal I14 assumes the same potential which is negative with respect to cathode 32, since the latter is connected to tap I15, and rectifier A is therefore non-conductive. When amplifier V is conductive, however, the potential drop across relay winding 82 opposes the potential difference between terminal 164 and tap H to the end that the potential of control electrode 33.becomes more positive with respect to cathode 32, and rectifier- A becomes conductive. The rectifier A is, therefore, responsive to the condition of amplifier V, becoming conductive only when the amplifier is conductive.

If no flame exists at the flame electrode,

control electrode 38 is substantially at cathode potential, and detector B is conductive. Amplifier V and rectifier A are therefore non-conductive, and the only potential between flame electrode 52 and the grounded burner 4 is that across resistor 40, which is due to the fiow of current from secondary winding I63 through resistors I12 and III in series. When a fiame appears between the burner and the electrode, its normal rectifying action allows more current to 110W from the electrode to ground than in the opposite direction. This causes the condensers 53 and 54 to become charged so that the control electrode 35 becomes negative with respect to the cathode 35, the output of detector 13 decreases, and that of amplifier V and rectifier A increases. When rectifier A becomes conductive, it substantially short-circuits the resistor 40 during the half cycles when terminal I65 of the transformer winding 183 is positive. During the alternate half cycles, however, a large potential exists across resistor 40, and a current flows along the circuit branch parallel to resistor 40, through resistors 45 and 41 and condensers 53 and 54, flame electrode 52, the fiame resistance, and ground. 'This pulsating current flow increases the charge on the condensers 53 and 54 so that the control electrode 38 becomes even more negative with respect to cathode 35. It is thus'seen that the action of the circuit is cumulative, and that once the detector output is reduced by the flame rectification, it is reduced still further by the regenerative action of the amplifier on the rectifier.

It has been found that this circuit is not critical with respect to the position of the flame electrode 52 in the fiame since when a fiame is initiated the outer parts of the flame must touch the electrode first as the flame is propagated in the direction of the flame electrode. These outer parts rectify in the proper direction to reduce the detector output slightly, as explained starts the rectifier operation, and biases the 26 flame electrode 52 positively with respect to ground, making the circuit insensitive to flame electrode position.

The modification of Fig. 14 is particularly desirable in applications where severe carbonization of the flame electrode 52 or of the burner 4 is obtained in that the circuit response to leakage paths between the flame electrode and ground produced by such carbonization is minimized.

That is to say, if the flame should become ex- 3 tinguished when a leakage path having anohmic resistance of the order of the normal fiame resistance exists, the negative potential applied on the control electrode 38 of detector B will be slightly decreased, and accordingly, the detector is rendered more conductive. This efiects a reduction in the current conducted by amplifier V and thereby a reduction in the current conducted by rectifier A. That reduction in rectifier current results in a decrease in the direct current voltage applied between the flame electrode and ground and thereby 'in a further increase in conductivity of detector B because of the reduction in potential drop across resistances 4'8 and 41 produced as a result of the direct current voltage decrease. This efiect is, therefore, seen to be a regenerative one, and accordingly, after a predetermined period determined by the circuit constants, the relay 83 will be deenergized and thereby the system shut down.

While in accordance with the provisions of the statutes I have illustrated and described preferred embodiments of the present invention, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that some features of the present invention may sometimes be used with advantage,

without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. An electric circuit for controlling a load device in accordance with the ma nitude of :1 variable impedance element, comprising a load device, a variable impedance element, an energizing circuit adapted to be connected to a source of alternating current of predetermined freouency, an electric discharge rectifier device having an input circuit including said impedance element and an output circuit. a connection between sa d load device and said output c rcuit including said energiz ng circuit, and means for intermittently impressin a substant ally un directional potential of constant magnitude and of the same frequency as said alternating current across said impedance element during the half cycles of sa d alternating current when said discharge rectifier dev ce is non conductive.

2. An electric circuit for controllin a load de vice in accordance -with the impedance of a flame, comprising in combination. a pair of spaced electrodes insulated from each other and adapted to be engaged by a flame, an energ zing circuit adapted to be connected to a source of alternating current of predetermined frequency, an electric discharge rectifier device having an input circuit including said fiame electrodes and an output circuit, a load device, a connection between said load device and said output circuit including said energizing circuit, and means for intermittently impressing a. substantially unidirectional poten-=' tial across said electrodes during the half cycles 

